Integrated circuit fabrication is only one example of a manufacturing process that requires close tolerances of the width of features produced on the wafer. For example, it is important that the width of etch lines during resist processing be controlled within certain critical dimension limits.
Today's wafer fabrication processes include real-time process control steps as a part of the manufacturing process. This permits appropriate adjustments to be made. Ideally, linewidth monitoring is on a wafer by wafer basis, and performed in-situ.
A number of existing techniques may be used to perform linewidth measurements. Techniques based on ordinary microscopes are satisfactory for use when the line widths are about 1.5 micrometers or larger. For submicron measurements, older methods use scanning electron microscopy. A problem with this method is that it is not real time or in-line, and thus is not conducive to wafer by wafer process control.
A recently developed linewidth measurement method uses test patterns in the form of diffraction gratings, which are placed in a test area of the wafer. Depending on whether the substrate being tested is transparent, the incident light is reflected or transmitted to generate diffraction beams. The gratings are monitored during fabrication by illuminating the grating with a monochromatic light, such as from a laser beam, and analyzing the resulting diffraction pattern to determine its linewidth. If the lines of the grating are made in the same manner as lines on the rest of the wafer, measurement of the grating linewidth can be used to infer the linewidths of the other lines on the wafer.
A linewidth measurement system using a diffraction grating is described in U.S. Pat. No. 4,330,213 to Kleinknecht, et al. The system obtains the intensity of first and second order light and uses the following equation to determine, the linewidth, lw: EQU lw=d/.pi. cos.sup.-1 (I2/I1).sup.1/2
where d is grating period (linewidth plus spacewidth), and I1 and I2 are measured intensities of the first and second diffraction orders.
The advantage of using intensity ratios is that many factors that affect intensity drop out of the equation. However, in practice, approximations such as the one in the preceding paragraph are insufficient for accurate measurements. Because the intensity ratio also depends on the ratio of line-to-space reflectivity, they are not accurate unless the lines and spaces being measured have very high contrast.
In addition to the problems of accounting for factors such as reflectivity, a problem with many existing diffraction methods is that they require the measurement of more than one order of diffraction. The availability of these intensity measurements is related to the width of the pitch for the lines being measured and the wavelength of the illuminating light. If conventional light sources such as visible or near ultraviolet are to be used, the existing methods require large pitch values.
A need exists for a method of measuring linewidths having small pitch values. The method should provide for in-situ monitoring as a part of a manufacturing process control.